Fuel cells have been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. One example of a fuel cell is the Proton Exchange Membrane (PEM) fuel cell. The PEM fuel cell includes a membrane-electrode-assembly (MEA) that generally comprises a thin, solid polymer membrane-electrolyte having a catalyst and an electrode on both faces of the membrane-electrolyte.
The MEA generally comprises porous conductive materials, also known as gas diffusion media, which form the anode and cathode electrode layers. Fuel, such as hydrogen gas, is introduced at the anode where it reacts electrochemically in the presence of the catalyst to produce electrons and hydrogen cations. The electrons are conducted from the anode to the cathode through an electrical circuit connected therebetween. Simultaneously, the hydrogen cations pass through the electrolyte to the cathode where an oxidant, such as oxygen or air, reacts electrochemically in the presence of the electrolyte and catalyst to produce oxygen anions. The oxygen anions react with the hydrogen cations to form water as a reaction product.
The MEA is generally interposed between a pair of electrically conductive contact elements or bipolar plates to complete a single PEM fuel cell. Bipolar plates serve as current collectors for the anode and cathode, and have appropriate flow channels and openings formed therein for distributing the fuel cell's gaseous reactants (i.e., the H2 & O2/air) over the surfaces of the respective electrodes. Bipolar plates can be assembled by bonding together two unipolar plates having the flow distribution fields formed thereon. Typically, bipolar plates also include inlet and outlet headers which, when aligned in a fuel cell stack, form internal supply and exhaust manifolds for directing the fuel cell's gaseous reactants and liquid coolant to and from, respectively, a plurality of anodes and cathodes.
As is well understood in the art, the membranes within a fuel cell need to have a certain relative humidity to maintain an ionic resistance across the membrane within a desired range to effectively conduct protons. During operation of the fuel cell, moisture from the MEAs and external humidification may enter the anode and cathode flow channels. As moisture is forced along the flow channels by a pressure of reactant gases, the highest concentration of water exists at the outlet regions of the bipolar plates where gas velocities are lowest. Water can accumulate and form a film in these regions, in a phenomenon known as stagnation. Stagnant water can block flow channels and reduce the overall efficiency of the fuel cell. A high degree of water accumulation or stagnation can lead to fuel cell failure.
Minimizing water stagnation has been possible, for example, by purging the channels periodically with reactant gas at a higher flow rate. However, on the cathode side, this increases the parasitic power applied to the air compressor and reduces overall system efficiency. Moreover, the use of hydrogen as a purge gas is not desirable for a number of reasons, including reduced economy, poorer system efficiency, and increased system complexity.
A reduction in accumulated water in channels can also be accomplished by lessening inlet humidification. However, it is desirable to provide at least some relative humidity in the anode and cathode reactants to hydrate the fuel cell membranes. Dry inlet gas has a desiccating effect on the membrane-electrolyte and can increase a fuel cell's ionic resistance. This method also negatively affects the long-term durability of the membrane-electrolyte.
There is a continuing need for a water removal mechanism that transports accumulating water from the flowfield channels to the exit of the fuel cell stack. Desirably, the mechanism does not reduce system efficiency or have a drying effect on the membrane-electrolyte of individual fuel cells.